1. Technical Field
The present invention generally relates to a fiber optic distributed temperature sensor system advantageously employing Raman scattered light.
2. Background Art
A fiber optic distributed temperature sensor arrangement using Raman scattered light measures a temperature distribution along the optical fiber by a following manner: Referring to FIG. 46 of the accompanying drawings, a light beam from one end of the sensor optical fiber, which light beam has a wavelength .lambda.o, a pulse width Tw and a pulse period Tp, is an incident light beam. Backward scattered light (reflected light) of two components of Raman scattered light produced in the optical fiber, one is a Stokes light having a wavelength .lambda.s and the other is an anti-Stokes light having a wavelength .lambda.as, are measured with a sampling time interval Ts, with a pulse light incident time t=0, as shown in FIG. 47. Then, time functions Ia(t) and Is(t) of intensity of the anti-Stokes light and Stokes light are respectively obtained as functions of the sampling time interval Ts. With the fact that the ratio of Ia(t)/Is(t) is purely a function of temperature and the fact that time until the scattered light produced (after the light pulse is input) at a position X in the optical fiber returns to the light pulse incidence end (reflected light measuring part) is 2X/Co. (Co: light velocity in the optical fiber), a temperature distribution measurement is carried out along the optical fiber. Here, the time width Tr (FIG. 47) for the reflected light measurement is 2L/Co (L: length of the optical fiber), and measured data in this time Tr give information of the temperature distribution. The backward scattered light measurements for the stoke light and antistoke light are carried out by a method similar to a method used for an OTDR (Optical Time Domain Reflectometry) device, which device is generally used for detecting breaking or fracture points of an optical fiber.
A temperature distribution measuring system using the optical fiber type temperature distribution sensor arrangement which takes advantage of Raman scattered light is, for example, used as follows: the optical fiber (sensor) is buried along the power cable to detect the temperature distribution along the longitudinal direction of the cable. This detection makes it possible to control an amount of power transmission and to find degradation of the cable by detecting the abnormally high portion in the temperature distribution. Also, it is possible to detect a fire when the sensor is used in a plant, building or tunnel.
FIG. 45 shows a conventional optical fiber type temperature distribution sensor arrangement using Raman scattered light. The sensor includes an optical fiber 6 for the sensor and a temperature distribution measuring system 2.
Pulse light from a pulse light source 4 for the sensor of the temperature distribution measuring system 2 is introduced into an optical fiber 6 (the sensor itself) via an optical fiber 5a and an optical turnout or optical wavelength division demultiplexer 5. A part of backward scattered light (reflected light) generated in the sensor optical fiber 6 is returned to the measuring system 2 and guided into an optical wavelength division demultiplexer 7 via the optical wavelength division demultiplexer 5 and an optical fiber 5b.
Part of the reflected light divided by the optical wavelength division demultiplexer 7 is introduced into the optical fiber 7a and this part enters an anti-Stokes light measuring system 30a such that the time function Ia(t) of the anti-Stokes light intensity is obtained from the light intensity of the above-mentioned part. The anti-Stokes optical measuring system 30a includes an optical filter 8a for the anti-Stokes light having a central wavelength .lambda.as, a light receiving device (detector) 9a and an averaging circuit 10a. On the other hand, another part of the reflected light divided by the optical wavelength division demultiplexer 7 is introduced into another optical fiber 7b and enters a Stokes light measuring system 30b such that the time function Is(t) of the Stokes light intensity is obtained from the light intensity of the above-mentioned other part. The stoke optical measuring system 30b includes an optical filter 8b for the Stokes light having a central wavelength .lambda.s, a light receiving device (detector) 9b and an averaging circuit 10b. A value of Ia(t)/Is(t) is obtained by a temperature distribution calculation circuit 11 to finally obtain the temperature distribution along the sensor optical fiber. Meanwhile, numeral 16 designates a device for displaying the temperature distribution.
However, according to the conventional optical fiber type temperature distribution sensor arrangement using Raman scattered light, the light emitted from the light source is guided to the sensor optical fiber via the optical wavelength division demultiplexer 5, and the scattered light generated in the sensor optical fiber is guided to the anti-Stokes light or Stokes light measuring system via the optical wavelength division demultiplexers 5 and 7 as well as the optical filter. Therefore, loss of at least 9 dB occurs at the optical wavelength division demultiplexers (3 dB as the light passes through one optical wavelength division demultiplexer), according to theortical calculations. This loss deteriorates the measuring accuracy.